As a next-generation communication system of the W-CDMA (Wideband Code Division Multiple Access) system or the HSDPA (High Speed Downlink Packet Access) system, an LTE (Long Term Evolution) system has been studied by 3GPP (3rd Generation Partnership Project) which is a standards body of the W-CDMA. More specifically, in the LTE system as a radio access system, an OFDM (Orthogonal Frequency Division Multiplexing) scheme and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme have been studied to be applied to the downlink communications system and the uplink communications system, respectively (see, for example, Non-Patent Document 1).
In the OFDM scheme, a frequency band is divided into plural sub-carriers having narrower frequency bands and data to be transmitted are mapped onto the sub-carriers. By using the orthogonality among the sub-carriers (i.e., independent from each other), the sub-carriers may overlap each other on the frequency axis, thereby enabling improving frequency use efficiency and achieving faster transmission data rate.
In the SC-FDMA scheme, a frequency band is divided into plural narrower frequency bands so that the divided frequency bands are allocated to plural mobile stations to communicate with the mobile stations. Further, in the SC-FDMA scheme, a range of the fluctuation of the transmission power may be made smaller; therefore, lower energy consumption of terminals may be achieved and a wider coverage area may also be obtained.
In both uplink transmissions and downlink transmissions of the LTE system, communications are carried out by allocating one or more physical channels that are shared among plural mobile stations. The channel shared among plural mobile stations is generally called a shared channel. In the LTE system, the shared channel in uplink and the shared channel in downlink are a Physical Uplink Shared Channel (PUSCH) and a Physical Downlink Shared Channel (PDSCH), respectively. Further, as transport channels, the shared channel in uplink and the shared channel in downlink are an Uplink Shared Channel (UL-SCH) and a Downlink Shared Channel (DL-SCH), respectively.
In the LTE system, by dynamically selecting mobile stations communicating using the shared channels, a highly-efficient best-effort-type communication system is achieved.
In the communication system using such a shared channel as described above, it is required to perform signaling (a signaling process) to allocate the shared channel to which mobile station with respect to each sub-frame (having one (1) ms period in the LTE system); and the control channel used for the signaling in the LTE system is called a Physical Downlink Control Channel (PDCCH) or a Downlink L1/L2 Control Channel (DL L1/L2 Control Channel). Further, the Physical Downlink Control Channel (PDCCH) is also used to transmit commands for transmission power control and the Acknowledgement Information (ACK/NACK) with respect to the Uplink Shared Channel (UL-SCH).
The information items transmitted via the Physical Downlink Control Channel (PDCCH) include, for example, a Downlink L1/L2 Control Channel Format Indicator (DL L1/L2 Control Channel Format Indicator), Downlink Scheduling Information, the Acknowledgement Information (ACK/NACK), Uplink Scheduling Grant, an Overload Indicator, Transmission Power Control Command Bit and the like (see, for example, Non-Patent Document 2).
Further, the Downlink L1/L2 Control Channel Format Indicator (DL L1/L2 Control Channel Format Indicator) may also be called a Physical Control Format Indicator Channel (PCFICH). Further, the Downlink Scheduling Information may also be called Downlink Scheduling Grant or Downlink Assignment Information. Further, the Downlink Scheduling Information and the Uplink Scheduling Grant may also be collectively called Downlink Control Information (DCI).
Further, the Downlink Scheduling Information may include information items regarding the Downlink Shared Channel (DL-SCH), such as allocation information of downlink Resource Blocks, an ID of a user equipment (UE) terminal (mobile station), the number of streams, information of Precoding Vector, data size, modulation scheme, information of an HARQ (Hybrid Automatic Repeat reQuest) and the like. Further, the Uplink Scheduling Grant may include information items regarding the Downlink Shared Channel (DL-SCH), such as allocation information of uplink Resource Blocks, the ID of a user equipment (UE) terminal (mobile station), the data size, the modulation scheme, uplink transmission power information, information of a Demodulation Reference Signal in an uplink MIMO and the like.
Currently, it is being studied that approximately forty (40) to approximately one hundred (100) bits are to be allocated to the Downlink Scheduling Information or the Uplink Scheduling Grant. This number of bits may be suitable when used in a high transmission data rate such as 10 Mbps through 100 Mbps but may not be suitable when used in a voice signal having a transmission data rate in a range between approximately 10 kbps and approximately 12 kbps. More specifically, if the number of bits of the Downlink Scheduling Information is 50, this transmission data rate (50 kbps, i.e., 50 bits per 1 ms) of the Downlink Scheduling Information exceeds the typical transmission data rate of the actual data signals (which is in a range between approximately 10 kbps and approximately 20 kbps), which may degrade the efficiency of the communications.
To overcome the problem, in the LTE system, with respect to user data having a substantially constant transmission data rate such as a voice service like VoIP, PoC, Speech service or the like, one scheduling scheme has been proposed to be used in which radio resources are repeatedly allocated every predetermined period instead of using a best-effort-type scheduling scheme in which the radio resources are dynamically allocated in order to obtain higher use efficiency (see, for example, Non-Patent Document 3). This proposed scheduling scheme is called Persistent Scheduling, Semi-persistent Scheduling or the like. On the other hand, the scheduling scheme normally used in which the radio resources are dynamically allocated is called Dynamic Scheduling or the like.    Non-Patent Document 1: 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006    Non-Patent Document 2: 3GPP R1-070103, “Downlink L1/L2 Control Signaling Channel Structure: Coding”    Non-Patent Document 3: 3GPP R1-060099, “Persistent Scheduling for E-UTRA,” January, 2006